In February 1984, the DTM converted from manual to automated platelet collection techniques, acquiring for this purpose four automated cell separation devices. During the next 10 years, 26 donors undergoing apheresis procedures on the Fenwal CS-3000 device experienced acute hypersensitivity reactions. Sixteen reactions occurred during plateletpheresis, and 10 reactions occurred during granulocytapheresis procedures. The purpose of this study was to identify the allergen responsible for these reactions. Using a combination of skin tests, radioallergosorbent tests, and basophil histamine release assays, specific IgE-mediated sensitization to ethylene oxide, a gas used to sterilize the plastic disposable apheresis kits, was found in 10 of 16 plateletpheresis donors and 8 of 10 granulocytapheresis donors experiencing reactions, but in none of 140 non-reacting controls. it is estimated that as many as 1.0% of all repeat apheresis donors may become sensitized to ethylene oxide and experience allergic symptoms during donation. Two of the 10 individuals experiencing reactions during granulocyte donations had positive skin tests to hydroxyethyl starch and appeared to become sensitized to this red cell sedimenting agent during apheresis. Although none of these reactions was life-threatening, the DTM is continuing to test apheresis donors for sensitization to ethylene oxide and hydroxyethyl starch, and to defer sensitized individuals from subsequent CS-3000 donations. The results of these studies were reported to the manufacturer of the CS- 3000 apheresis device and to CBER/FDA. Similar reactions were subsequently reported by another group in plasmapheresis donors. As a result of these reports, the manufacturer of the CS-3000 disposable apheresis kits changed the process by which the kits are sterilized from an ethylene-oxide based system to one involving predominantly gamma sterilization. The manufacturer states that kits manufactured after 1995 no longer contain significant ethylene oxide residuals. The most recent hypersensitivity reaction to occur in a DTM donor was during a granulocytapheresis procedure in August 1995. No reactions have been noted since then, although about 2000 CS-3000 platelet- and granulocytapheresis donor procedures continue to be performed per year.